1. Field of the Invention
The present invention relates to the purification of crude wet-process phosphoric acid, and, more especially, to a process for preparing a colorless, or but slightly colored phosphoric acid from an uncalcined phosphate by means of a particular decolorizing and liquid-liquid extraction sequence.
2. Description of the Prior Art
The preparation of purified phosphoric acid or phosphate, comprising the digestion or acidulation of phosphate rock with a strong acid, such as sulfuric acid, followed by liquid-liquid extraction of the crude liquid resulting upon filtration (said crude liquid containing numerous anionic and cationic impurities and being highly colored) utilizing an essentially water-insoluble organic extractant, e.g., alkyl phosphates or C.sub.4 -C.sub.9 aliphatic alcohols, is known to this art. Such procedure is typically followed by washing the separated organic phase containing the phosphoric acid with water, and, ultimately, by re-extraction of the acid either with water or an aqueous phase containing alkaline ions, to afford a substantially purified phosphoric acid or phosphate solution. Compare U.S. Pat. Nos. 3,607,029 and 3,767,769, assigned to the assignee hereof.
It too is known to this art that the phosphate rock starting material contains numerous impurities organic in nature, which originate either from the mineral deposit itself, from the additives introduced or incorporated during the physical pretreatment of the phosphate rock, and/or, possibly, from the organic additives introduced or incorporated upon digestion or acidulation with the strong acid (antifoaming agents, and the like). It will of course be appreciated that the amount of organic carbon present in the phosphate rock varies according to the origin of said rock. After the digestion of the rock, typically by sulfuric acid, and the filtration of the gypsum therefrom, a raw or crude liquor is recovered, which, as a function of the original content of organic carbon in the starting material rock, has a coloration ranging from a greenish-yellow to opaque muddy brown. The organic carbon content (determined via the amount of CO.sub.2 obtained after oxidation in this solution) may vary significantly, for example, between 0.013 and 0.04% by weight of the solution (0.04 to 0.12%, expressed as P.sub.2 O.sub.5) in a phosphate originating from Morocco, e.g. Khouribga, or Togo, to a value as high as 0.5% expressed as P.sub.2 O.sub.5 for a Florida phosphate.
For certain applications, such as the manufacture of fertilizers the immediately aforesaid, while not particularly attractive, is not technically detrimental. However, this is of course not true for the case of preparation of a purified acid destined for animal or human nutritional purposes, i.e., food-grade acid.
The application on an industrial scale of continuous liquid-liquid extraction processes to such crude acids containing organic matter by means of the usual organic solvents, including recycling to extract the phosphoric acid, is difficult to effect in actual practice, or may even be impossible, depending upon the amount of colored organic material present in the beginning raw acid and the particular mode of re-extraction employed (by water or by a phosphate solution). And in the event that an emulsion of the plural phases generated by reason of the presence of the noted organic material during the intimate admixture of the crude aqueous phase with the organic extractant, remains stable upon cessation of the agitation, it is practically impossible to effect decantation of the phases, i.e., phase separation, under conditions compatible with the normal conditions of the continuous operation of an industrial plant. Furthermore, even if it is possible to attain phase separation over a prolonged (but not industrially feasible) period of time, a zone having a muddy consistency appears in the vicinity of the interface of the phases, which zone consists of blackish, gelatinous floccules. This phenomenon is enhanced with increase in the organic carbon content of the crude liquors. Similarly, the solvents are imbued with a very deep brown color when enriched in organic matter, so that after a certain number of recycles of the solvent, same can no longer be used.
As a result of the foregoing, especially with respect to the Florida phosphates which are quite rich in organic material, continuous liquid-liquid extraction becomes strictly impossible on an industrial scale, while with certain other phosphates containing but little organic matter, such operation may indeed be performed in an acceptable manner. However, even with these latter phosphates, an accumulation of gelatinous products occurs at the phase interface, particularly when certain solvents (tributyl phosphate, for example) are utilized and when the re-extraction is carried out not with water but with a monosodium phosphate solution. Furthermore, the content in organic matter of the phosphate solution is too high to give rise to sufficiently white sodium phosphate or polyphosphate solutions (for example, a sodium tripolyphosphate solution).
The aforesaid technical disadvantages and drawbacks have long been dealt with in this art by first reducing the amount of organic material in the starting material rock. It is known in this respect, for example, to calcine the rock at temperatures between 700.degree. C. and 1000.degree. C. prior to digestion, for a period of time ranging from a few minutes to about one hour. It is thus possible to effect a liquid-liquid extraction of the crude liquor under industrial conditions and to recover a purified phosphoric acid.
Nevertheless, the above-described calcining treatment affects only the suitability of the crude acid for downstream extraction and does not ultimately yield a re-extracted acid that is colorless after concentration; rather, fraction of the organic material remaining after the calcination of the rock, possibly in a colorless state, colors the aqueous acid after re-extraction with water, and during the final operation of concentration at an elevated temperature, such that same becomes unacceptable in the human and animal food industries, i.e., is not an acceptable food-grade additive. The result is that the already purified acid must be subjected to a further decolorization treatment, or the residual organic material must then be eliminated by oxidation with a powerful oxidant, as described in U.S. Pat. Nos. 2,013,970 and 4,044,108, and German Pat. No. 884,358, (wherein the more or less concentrated phosphoric acid is heated in the presence of a chlorate, hydrogen peroxide or another oxidant), to obtain a colorless or slightly colored acid suitable for use in the foodstuff industry.
However, the preliminary calcining treatment is expensive and highly energy intensive. Furthermore, in an industrial installation for the manufacture of wet-process phosphoric acid it frequently transpires that only a fraction of the raw acid is destined for ultimate preparation of the purified acid, while the remaining fraction, generally the larger, is destined for ultimate production of various fertilizers; consequently, in order for the installation or facility to operate continuously, while continuously producing the aforesaid two types of acid, it is necessary to calcine all of the starting material phosphate rock, and this proportionally increases the cost of calcination per ton of purified acid produced.
A possible continuous process for the extraction of a wet-process crude phosphoric acid obtained by sulfuric acid digestion of uncalcined mineral is described in published French application No. 2,132,203, at pages 9-10 (U.S. Ser. No. 129,075, filed Mar. 29, 1971 in the names of Peter Tao-I Chiang and J. D. Nickerson). These applicants also have noted that extraction of the uncalcined mineral by means of an aliphatic alcohol gives rise to an increase in the tar content of the extractant, in a continuous process characterized by solvent recycling, to such an extent that the process becomes useless. This drawback is obviated by a supplementary solvent purification stage effected with alkali, or by distillation to reduce the tar content to an acceptable level. However, it remains true that the lengths of time suggested for decantation would be incompatible with a truly industrial process.
Accordingly, serious need exists in this art for a continuous process yielding a purified phosphoric acid by liquid-liquid extraction including solvent recycling, and with the resultant acid being colorless or but slightly colored, without concomitant requirement for calcination.